In the field of electric power steering systems, it is known to improve the steering feel of a vehicle by appropriately controlling the force that urges the steering wheel toward the neutral position after making a turn.
The electric power steering system disclosed in Japanese patent laid open publication (kokai) No. 2003-220964 (patent document 1) comprises a power source for providing an actuating force to a vehicle steering mechanism, a steering angle detecting means for detecting the angular position of the steering wheel, a vehicle speed detecting means for detecting a vehicle speed and a self-aligning torque compensation means for compensating for any excess or deficiency in the self-aligning torque of the vehicle according to the steering angle detected by the steering angle sensor and the vehicle speed detected by the vehicle speed sensor, wherein the self-aligning torque is maintained at an appropriate level at all times by increasing a correction gain for the self-aligning torque when the vehicle speed is low and reducing the correction gain substantially to zero when the vehicle speed is medium or high.
According to another proposal disclosed in Japanese patent laid open publication (kokai) No. 2005-271877 (patent document 2), a steering system is provided with an actuator that can control the steering angle of road wheels, and controls the steering angle when returning the steered wheels back to the neutral position according to the magnitude of the steering angle immediately preceding the returning of the steered wheels.
Having conducted research in this field, the inventors have discovered that the steering feel of an electric power steering system that provides an assisting steering torque can be significantly improved by determining the returning force that urges the steering wheel toward a neutral position by taking into account if the direction of the steering torque agrees with the rotational direction of the steering wheel or not.
The prior inventions disclosed in patent documents 1 and 2 were directed to the control of the returning force that urges the steering wheel toward the neutral position, but did not consider the relationship between the direction of the steering torque and the rotational direction of the steering wheel. Therefore, according to the prior art, there was a problem that the returning force of the steering force tends to be excessive when the direction of the steering torque agrees with the rotational direction of the steering wheel, and this impaired the steering feel and/or unduly increased the effort required for effecting a steering action.